So the 2 Arduino are connected to 2 USB ports and the PC software treats them as 2 separate devices?
I guess that could be ok. Mostly, beginners believe they need to use 2 Arduino in the same project for the wrong reasons, making the project more difficult than it needs to be. In you project, I don't know, perhaps that is the best way.
Yes, you can use the same PSU. Use as many LEDs as you need but be sure that the PSU has capacity for all the LEDs that may be on at a single time, and be sure to have a common ground between both Arduino and both sets of LEDs.